In conventional radiography, visible images are formed by X-ray exposure of a film. The perceptibility of details on such an X-ray film is dependent on the one hand on the detail contrast and on the other hand on the density.
"Density" is to be understood herein as the common logarithm of the quotient of the quantity of light incident on the X-ray film and the quantity of light transmitted by the X-ray film. The meaning of this term is the same as that of the terms "blackening" or "optical density" as used in relevant literature. The density of a film increases as a function of the common logarithm of its exposure, ignoring solarization effects. This dependency of the density on the logarithm of the exposure will also be referred to as "density function" hereinafter.
The contrast C is referred to herein as the differential quotient of the density function, i.e. C=dD/d(log B), where D is the density and B the exposure. The dependency of this (detail-) contrast on the logarithm of the exposure will be referred to hereinafter as "contrast function". Thus, the density function and the contrast function of a film are inseparably linked, i.e. they are correlated.
In digital X-ray exposure systems the X-ray exposure does not yet yield a visible image, but rather a data field consisting of digital input image values which are dependent on the exposure. This data field can be converted into a visible image by means of a suitable output unit, for example a laser imager or a monitor. As is known from the book by Christensen "Introduction to the Physics of Diagnostic Radiology", 3rd Edition, Lea & Febiger, Philadelphia, 1984, the user can preset the increase of the density function and its position by so-called windowing. However, density function and contrast function still remain interdependent (the contrast corresponds to the slope of the density function).
EP-OS 482 712 which corresponds to U.S. Pat. No. 5,357,549, discloses a method of converting digital input image values into a visible image in which the dynamic range in the large-area image zones is compressed while the detail contrasts are maintained. To this end, the input image values are subjected to low-pass filtering. The low-pass image values thus formed are transformed in conformity with a compensation function which produces positive image values for small low-pass image values and negative image values for large low-pass image values. These images values are superposed pixel-by-pixel on the input image values.
It is a drawback that the compensation function to be preset by the user on the one hand changes the brightness or the density of the output image, but on the other hand also affects the image in a manner which cannot be directly anticipated by the user.
Furthermore, DE-PS 29 52 422 discloses a so-called Unsharp Masking method in which high-pass image values are derived from the digital input image values of an X-ray exposure, said high-pass image values being weighted by a weighting factor and then superposed on the input image values. The weighting factor may be a constant, but may also be varied in dependence on the input image values or in dependence on low-pass image values derived therefrom.
In both methods the ratio of the small structures in the image (or the high spatial frequency components) to the large structures in the image (or the low spatial frequency components) is modified in comparison with the non-processed input image, that is to say in such a manner that the small structures are emphasized. This means that the contrast function and the density function are no longer correlated when the contrast function is related to the image areas with the higher spatial frequencies and the density function is related to the image areas with the low spatial frequencies. However, it is not clear to the user whether and how the parameters preset the user, i.e. the compensation function or the weighting factor, influence the contrast and the density of the visible image.